Iodine cycle incandescent lamps



y 5, 1954 c. B. COLLINS ETAL 3,132,278

IODINE CYCLE INCANDESCENT LAMPS Filed Sept. 18, 1961 ITWVEETWTOT'SI CLi I FOT'd B. CoLLins Edward G. ZLlbLGT" United States Patent Ofi ice 3,132,278 Patented May 5, 1964 3,132,278 IGDINE CYCLE INCANDESCENT LAMPS Clifford B. Collins, Cleveland Heights, and Edward G. Znhler, Chagrin Falls, Ohio, assignors to General Electric Company, a corporation of New York Filed Sept. 18, 1961, Ser. No. 138,863 7 Claims. (Cl. 313178) This invention relates generally to electric incandescent lamps comprising a tungsten filament sealed in a bulb or envelope of light-pervious material containing a filling of inert gas, and more particularly to so-called iodinecycle lamps which contain a small proportion of iodine vapor which makes possible operation of the lamp at a high temperature and efficiency for a long useful life with virtual freedom from blackening of the bulb throughout life.

The blackening of the bulbs of ordinary incandescent lamps is the result of sublimation or volatilization of the filament which increases with filament tempearture. Although the eficiency of a filament increases very rapidly with increasing temperature, the resultant blackening places a practical limitation on the temperature at which it may be operated and still have a reasonable life. The effect of such bulb blackening is minimized to an appreciable extent by employing bulbs of comparatively large size to distribute the blackening over a large area and also by the presence of an inert gas such as nitrogen, argon, krypton, xenon etc. to minimize filament vaporization. I

'In the iodine cycle lamp, such as disclosed and claimed in U.S. Patent No. 2,883,57lFridrich et al., such bulb blackening is virtually eliminated by employing a relatively small compact bulb and adding a small quantity of iodine vapor to the gas filling in the bulb. The iodine functions as a regenerative getter in association with the tungsten filament by combining with evaporated tungsten at the hot bulb Wall to form tungsten iodide which migrates to the vicinity of the hot tungsten filament where it is dissociated and the tungsten is returned to the filament. The tungsten-iodine cycle operates throughout the life of the lamp, which may be hundreds or thousands of hours, to keep the bulb walls free of blackening.

As pointed out in the aforesaid Patent 2,883,571, the iodine cycle performs effectively provided the lamp is kept free of impurities. One such impurity is iron which is normally present in the tungsten filament. However, the iron content can be kept to a safe limit, preferably not more than about 002% by weight, by etching the filament in suitable acids to remove excess iron which is otherwise evaporated during lamp operation to form a deposit on the bulb wall which combines with the iodine to form iron iodide and thereby destroy the regenerative cycle.

-By properly baking and evacuating the bulb and degasing the metal parts including the filament, the iodinetungsten cycle functions efficiently. However, the elimination of impurities such as oxygen and water vapor may require careful and prolonged processing of the lamp, particularly in lamps of larger size or longer length, for example when the length of a tubular lamp exceeds about three times its diameter. The oxygen and water vapor are evolved from the bulb or other lamp parts or are formed by chemical combination between oxides inadvertently introduced into the lamp and hydrogen introduced into the gas filling or the sealing fires.

Such processing of the lamp is costly, and it is therefore an object of the present invention to provide a relatively simple and effective means of ridding the lamp of the effects of oxygen and Water vapor.

In accordance with the invention, it has been discovered a of hydrogen, which is normally the case.

' lamp, the situation is quite different.

that the adverse effects of water vapor and oxygen in an iodine cycle lamp may be obviated by proper use of carbon, namely by the inclusion of a small quantity of solid carbon concentrated at one or more locations where it will react with oxygen, that is, where the temperature is preferably of the order of 1000 C. or more, but can be as low as 500 C. The oxygen is thereby tied up as carbon monoxide which has been found not to be harmtul in small quantities. However, the presence of carbon on the hot tungsten filament or as a film covering the inner wall of the lamp bulb must be carefully avoided, although it may be placed on the inner lead wire portions 5 or on the relatively cooler ends of the filament 6 which are below about 1500" C. Otherwise it leads to filament sagging and disruption of the iodine cycle. In other words, carbon has been found to be highly beneficial when properly employed but very harmful when used improperly.

In an incandescent lamp not containing iodine vapor, when water is present in the lamp atmosphere a well known phenomenon, referred to as the water cycle, occurs. The water reacts at the hot tungsten filament to form tungsten oxide and hydrogen. The tungsten oxide evaporates from the filament, part of it depositing on the bulb wall to cause blackening, which accumulates. Part of the tungsten oxide diffuses to adjacent turns of the coiled filament where it decomposes, freeing oxygen which again reacts with the filament. The hydrogen released when water reacts with the hot filament is dissociated from molecular to atomic hydrogen. The 'very reactive atomic hydrogen diffuses to the bulb wall where it reduces the silica of the quartz or glass bulb, taking oxygen back to the filament. The cycle continues because of the presence of the hydrogen in the lamp which causes oxygen to be continuously supplied to the filament.

' The cycle can be stopped only by removing all hydrogen from the atmosphere of the lamp. Various methods, including the use of getters such as tantalum etc., are in common use to stop Water cycling in many incandescent lamps. However, it will be noted that such getters cannot be employed in iodine cycle lamps because of their reaction with the iodine.

j When oxygen orwater is present in an iodine cycle As before, oxygen or water vapor will react with the tungsten filament, forming tungsten oxide which evaporates, part of it decomposing at adjacent turns of the filament coil to deposit tungsten and free the oxygen, the remainder diffusing to the bulb wall where it deposits as tungsten oxide. However, the tungsten-iodine cycle now operates by the formation of tungsten iodide which returns to the filament and decomposes, depositing'tungsten back on the filament. Darkening of the bulb wall because of the presence of oxygen or water is not a problem.

Hydrogen from decomposed water or other sources is tied up as hydrogen iodide. At the hot filament, the hydrogen iodide may decompose, but free atomic hydrogen leaving the region of the hot filament recombines with iodine to again form hydrogen iodide before reaching the bulb wall. Therefore free atomic hydrogen does not exist as long as the amount of iodine is greater than the amount Under these conditions the hydrogen as hydrogen iodide is unable to reduce the quartz or glass at the bulb wall and, therefore, cannot provide a fresh supply of oxygen which can attack the hot tungsten filament. Therefore, the water cycle does not operate in the usual manner in incandescent lamps Containing iodine.

' What does occur when traces of oxygen or water are present, is a type of oxygen-tungsten-iodine cycle. Tungsten oxide on the wall reacts with iodine vapor to form tungsten iodide which diffuses to the filament where it is decomposed, depositing tungsten. By this process the oxygen from the tungsten oxide is freed to again attack the filament. If the oxygen is not removed from the lamp, the filament will be destroyed. In accordance with the invention, the cycle is stopped by the addition of carbon which reacts with the oxygen at an adequately high temperature to form carbon monoxide which is not harmful in low concentrations.

It has been proposed, nearly forty years ago, that ordinary incandescent lamps of that period be freed of water vapor by applying carbon to the filament. However, as indicated above, the addition of carbon to the filament of present day lamps has been shown to be definitely harmful because of the much higher filament temperature which causes rapid reaction of the tungsten and carbon, resulting in sagging of the filament and lamp failure. Only by restricting the added carbon to the ends of the filament which operate at a much lower temperaiure is it possible to avoid excessive sagging or lamp failure. It will be noted that in the earlier lamps the operating temperature of the filament was in the range of about 2300 to 2450 K. (corresponding to efiiciencies of 9 to 10 lumens per watt). In contrast thereto, the filament of the present day iodine cycle lamps operates at a temperature in excess of 2500 K., for example about 2850 K. (21 to 22 lumens per watt) or higher.

It was, furthermore, not predictable that carbon would serve as a getter for oxygen in the iodine cycle lamp. There was no reason to believe that combination of carbon and oxygen would take place preferentially over the water cycle where the carbon was localized. There was also the matter of possible reaction of carbon with molec ular or atomic iodine. For example, the formation of C1 by the reaction of carbon with iodine was of concern since conceivably the compound CL, could be a mechanism for transporting carbon to the filament and eventually causing sagging of the filament and failure of the lamp. Furthermore, the same mechanism of transport of carbon as C1 might operate to interfere with the desired reaction between tungsten and iodine at the bulb wall.

An embodiment of the invention is illustrated in the accompanying drawing wherein:

FIG. 1 is a side view of a lamp comprising the invention, and

FIG. 2 is a section taken along the line 22 in FIG. 1.

Referring to the drawing, the lamp shown therein for purposes of illustration is of the type disclosed and claimed in the aforesaid US. Patent 2,883,571. It comprises a tubular bulb or envelope 1 of light-transmitting material such as glass, quartz, alumina, etc., having its ends compressed to form flattened press or seal portions 2 in which are hermetically sealed and embedded the lead-in conductors or wires. With a bulb 1 of quartz, the lead wires preferably each comprise an outer section 3 of molybdenum wire, an intermediate section 4 of thin molybdenum foil, and an inner section 5 preferably of tungsten. As axially extending filament 6 of coiled tungsten wire is connected at its ends to the inner end portions 5 of the lead wires. In an elongated lamp, as illustrated herein, the filament 6 is supported intermediate its ends by tungsten wire spirals 7 engageable with the inner wall of the bulb 1. The bulb 1 contains a filling of inert gas at a substantial pressure, for example argon at a pressure of some 600 to 30,000 mm. Hg or higher, and a small quantity of iodine, preferably between about .01 and 1 micromole per cc. of bulb volume and which is all vaporized during operation of the lamp. The iodine cycle will still operate satisfactorily at even higher concentrations of iodine.

In accordance with the invention, solid carbon is introduced into the lamp in one or more concentrated locations where it will be heated to a temperature of the order of 1000 C. to combine chemically with oxygen or water vapor present in the lamp to form carbon monoxide. A

convenient location is on one or more of the filament supports 7, as indicated at 8 in FIG. 2 where the carbon is located sufiiciently close to the filament 6 to be heated to the desired temperature.

Prior to the assembly of the filament 6 and its associated support and lead wire into the bulb 1, the carbon is conveniently applied to the support 7 in the form of aquadag which is essentially a viscous suspension of very fine particles of colloidal graphite in water. The amount of carbon is not critical, it being necessary only to supply sufiicient carbon to combine with the oxygen or water vapor present in the lamp atmosphere. An excess of carbon on the support is not harmful. In fact an excess is desirable to assure clean-up of oxygen and water vapor during life of the lamp. Due to variations in processing procedures, resulting in variations in amounts of oxygen and water vapor present in the lamp, the amount of carbon should be determined empirically for a given size lamp and a given processing procedure.

By way of example, in a 11500 watt lamp of the type shown in the drawing, having a quartz bulb 1 of about 7.75 mm. inside diameter and an internal length of about 210 mm., good results have been obtained by applying to five of the supports 7, coatings 8 of aquadag at a location about 3 mm. from the filament and over an area about 2 mm. long, covering half the diameter of the support 7 facing the filament, and about 1 mil thick. The total amount of carbon may be about .10 milligram. The coating 8 may be applied by means of a fine brush wet with the aquadag suspension.

In a 500 watt lamp having the same diameter and an internal length of about 75 to mm., and provided with two filament supports 7, it may be sutficient to provide the dab 8 of aquadag to only one of the supports, although it is preferably applied to both supports to efi'ectively clean up the oxygen. The total amount of carbon is about .02 milligram on each support wire.

Except for the addition of the carbon, the lamp may be processed by generally familiar lamp making techniques. Thus, prior to assembly, the filament coil 6 (still carrying the molybdenum mandrel on which it was Wound) may be heated in a hydrogen furnace to degas it and set the coil. After the mandrel has been dissolved, the filament is assembled with the supports 7 and lead wires 3, 4, 5, the carbon is applied to the supports 7, and the filament assembly is sealed into the bulb 1. The lamp is then evacuated through an exhaust tube, the residue of which is shown at 9; flushed with a suitable inert gas such as argon; baked; preferably again evacuated and flushed; filled with the final gas filling of argon and dry iodine vapor; and tipped off or sealed at 9. Upon subsequent energization of the filament, the carbon at 8 reacts with residual oxygen and water vapor in the lamp to form carbon monoxide which is not harmful, and any hydrogen present in the lamp atmosphere is tied up by the iodine as hydrogen iodide.

As an example of the efiicacy of the carbon, whereas 35% of a group of lamps made without carbon were defective due to water or oxygen cycling, the addition of carbon reduced the ratio of defective lamps to about 1%.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric incandescent lamp of the iodine-cycle type comprising a sealed bulb of light-transmitting material containing an incandescible tungsten filament, a filling of inert gas and a small amount of iodine vapor in said bulb effective to function as a regenerative getter returning vaporized tungsten to said filament, and a localized quantity of carbon within said bulb in at least one location removed from said filament except at the relatively cooler ends thereof but heated thereby to a temperature sufficient to react chemically with oxygen and water vapor.

2. A lamp as set forth in claim 1 wherein the location of said carbon is such that the said temperature to which it is heated is about 1000 C.

3. A lamp as set forth in claim 1 wherein the material of said bulb is quartz.

4. A lamp as set forth in claim 1 including at least one support member of tungsten mounted to support said filament and wherein the carbon is carried by said support member at an area thereon adjacent said filament.

5. In the manufacture of an electric incandescent lamp of the iodine-cycle type comprising a sealed bulb of lightpervious material containing an incandescible tungsten filament, a filling of inert gas and a small quantity of iodine vapor, the steps which comprise introducing into the lamp a localized quantity of carbon in at least one location adjacent to but removed from said filament except at the relatively cooler ends, thereof, and energizing said filament to heat it to incandescence and to also heat said carbon to a temperature sufficient to react chemically with oxygen and water vapor.

of tungsten mounted to support the filament and wherein the carbon is applied to at least one of said support members at an area thereon adjacent said filament.

7. An electric incandescent lamp of the iodine-cycle type comprising a compact sealed tubular bulb of lighttransmitting material containing an axially extending tungsten filament, at least one tungsten wire support member engaging said filament at a point intermediate its end and engageable with the bulb wall, a filling of inert gas and a small amount of iodine in said bulb effective to function as a regenerative getter returning vaporized tungsten to said filament, and a localized quantity of carbon on said support member at an area thereon adjacent said filament and efiective to react chemically with oxygen and water vapor in said bulb.

Andrews Mar. 2, 1937 Fridrich et a1 Apr. 21, 1959 

1. AN ELECTRIC INCANDESCENT LAMP OF THE IODINE-CYCLE TYPE COMPRISING A SEALED BULB OF LIGHT-TRANSMITTING MATERIAL CONTAINING AN INCANDESCENT TUNGSTEN FILAMENT, A FILLING OF INERT GAS AND A SMALL AMOUNT OF IODINE VAPOR IN SAID BULB EFFECTIVE TO FUNCTION AS A REGENERATIVE GETTER RETURNING VAPORIZED TUNGSTEN TO SAID FILAMENT, AND A LOCALIZED QUANTITY OF CARBON WITHIN SAID BULB IN AT LEAST ONE LOCATION REMOVED FROM SAID FILAMENT EXCEPT AT THE RELATIVELY COOLER ENDS THEREOF BUT HEATED THEREBY TO A TEMPERATURE SUFFICIENT TO REACT CHEMICALLY WITH OXYGEN AND WATER VAPOR. 